NAME

       triangle - Triangle reference element

DESCRIPTION

       The triangle reference element is

               K = { 0 < x < 1 and 0 < y < 1-x }

NUMBERING

             y

             2
             | +
             |   +
             |     +
             |       +
             0---------1   x

       Curved  high  order  Pk  triangles (k >= 1), in 2d or 3d geometries, are supported.  These
       triangles have additional edge-nodes and face-nodes.

THESE NODES ARE NUMBERED AS

        first vertex, then edge-node, following the edge numbering  order  and  orientation,  and
       finally  the  face  internal  nodes,  following  the triangle lattice.  See below for edge
       numbering and orientation.

             2               2               2
             | +             | +             | +
             |   +           7   6           9   8
             5     4         |     +        10 14  7
             |       +       8   9   5      11 12 13 6
             |         +     |         +     |         +
             0-----3-----1   0---3---4---1   0--3--4--5--1
                   P2              P3              P4

IMPLEMENTATION

       const size_t dimension = 2;
       const Float  measure = 0.5;
       const size_t n_vertex = 3;
       const point vertex [n_vertex] = {
               point(0, 0),
               point(1, 0),
               point(0, 1) };
       const size_t  n_edge = 3;
       const size_t edge [n_edge][2] = {
               { 0, 1 },
               { 1, 2 },
               { 2, 0 } };